Pretty traces of light!

by Stephanie Chasteen on February 1, 2009

Welcome to the first post at the new blog location!  Now that my webhome is established I can start posting more regularly.  Geek on! 😎

Sebastien Martin of the Exploratorium has been working with what you can do with light traces — basically, tracing out the motion of something through space using light (say, by attaching an LED to the object and taking a time-lapse picture of it) for a while. He did some stuff with the science of baseball showing the path of a baseball through space, and has also posted some great pix on Flickr. Here is what he says about how to make these traces:

All the pictures were taken with a regular digital camera set to an exposure time of 0.5 to 3 seconds. The lines you see were created by LED lights attached to moving objects.

You can use any small light source to make the trace of a moving object visible (bicycle light, flash light, key light). Just attach the light to the object, and make sure the room is completely dark. Then take a long exposure picture of the moving light using your Digi Cam.

It’s also fun to go further and analyze the speed of the object! To do that, use a fast blinking LED light (such as the Inova pulsed LEDs you can buy at Target Stores for $7). The distance between the dots is a measure of speed.

In fact, Sebastien’s whole Flickr library is a source of amazingly creative science stuff.

(photo from Sebastien Martin)

{ 4 comments… read them below or add one }

Ave October 13, 2011 at 9:23 pm

I am looking for a specific answer to LED light behavior. On vehicle rear lights, some are being made with LEDs now.
While driving on the highway one night, I was passed by a vehicle which must have had this type of rear light. Glancing over to the vehicle and then back to my own lane produced a ‘repeat trace effect’ of the light, with the image getting smaller along the trajectory. The image of the rear lights repeated itself about 20 times along a swoop shape (similar to the Nike swoop), and then disappeared. This image lasted only briefly, but certainly long enough for me to remark on it and attempt (successfully) to repeat the experience, till the vehicle was about half a mile away.
I want to know if this is appropriate behavior for this type of light, or should I get myself to the doctor?
Thank you.

Stephanie Chasteen October 19, 2011 at 4:04 pm

I don’t think you need to get yourself to the doctor — it just sounds like what’s called a “positive afterimage” of the LED light. The LED is either blinking on and off rapidly, resulting in a repeating trace image of the light, or the eye is somehow not seeing a continuous after-image streak but rather a series of discrete after images. This after-image is seen as a streak since you’re moving your eye from that vehicle to the point in front of your own vehicle. The swoop shape is probably due to the way that you move your eyes (sounds like you move them down then back up, probably unconsciously) as you move your eye away from the original source. I believe the changing size of the image might be because you are moving from looking at something nearby (the taillight) to something far away (the road in front of you), and so your brain perceives the after-images in front of the road to be smaller in comparison to the other images in front of you. I know that’s a confusing explanation in that last sentence — I can write more if you want — the basic idea is that the brain perceives size to be relative to the distance of an object. The same size object a mile away, your brain knows to be larger in real life (an elephant) than the same size object right near by (a mouse).

Hope that helps!

Ave November 4, 2011 at 3:35 pm

Thank you Stephanie! I have had a few other experiences with light and did see an ophthalmologist. He diagnosed a normal aging condition where the gel of the eyeball pulls away somewhat from the back of the eye, causing light images to be repeated. What seemed odd to me was that the repeated image was colored, and not in the ‘negative effect’ way that a person can cause themselves by looking at a bright image, and having it repeat in the opposite color when looking away. A particular incident was a white chandelier in a fully daytime lit conference room, repeating back to me in fluorescent pink. This occurred in Jan. 2011. The LED rear car light event happened just prior to me blogging you.
On another topic – would you know of resources for a 50 year old to learn algebra and physics in an environment that would have plenty of dialogue and personal coaching? I am so interested in a variety of topics that keep pulling me in that direction – but I had trouble in high school and never success.
Thank you 🙂

Stephanie Chasteen November 7, 2011 at 4:23 pm

Wow, how interesting, Ave. I’m glad that you found out what the issue was.

Just FYI, “positive afterimages” aren’t in the opposite color from what the original light source is — above, you describe that you were surprised that it wasn’t in the opposite color. That would be a “negative afterimage.” Look them up on wikipedia. Your issue obviously wasn’t either one, but just so you know, you *can* see a bright image in the same color, caused by the light sensitive cells continuing to fire after you look away from the object. Vision is really weird.

As for learning resources — probably your local community college would be the best. There is also a nice online resource (which costs) called ALEKS, might be worth looking into.


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